This application is based on and claims the priority date of German Application No. 101 42 236.9, filed on Aug. 29, 2001, which is incorporated herein by reference.
The invention relates to an improved method for determining the reducing agent concentration (NH3) in the exhaust-gas flow of an internal combustion engine.
Nitrogen can be removed from diesel engine exhaust gases by using the selective catalytic reduction (SCR), with ammonia (NH3) as reducing agent. For this purpose, ammonia is carried along in the vehicle either directly or in the form of a compound from which the ammonia is obtained. For the catalytic reduction reaction, ammonia is metered into the engine exhaust gas at a specific ratio to the NOx content that is momentarily present in the exhaust gas. A stoichiometric NH3xe2x80x94NOx ratio must exist in order to achieve the maximum possible NOx conversion. A lower ratio leads to a lower conversion while a higher ratio leads to a so-called xe2x80x9cNH3 breakthrough.xe2x80x9d The optimum use of the nitrogen-removing catalytic converter is achieved with the aid of a zeolitic NH3 gas sensor, which is installed as a control element or as an NH3 breakthrough sensor in the exhaust-gas flow behind the catalytic converter. The NH3xe2x80x94NOx ratio can thus always be adjusted to the maximum possible NOx conversion.
The use of a zeolitic NH3 gas sensor requires that a connection be found between the measuring variable and the NH3 concentration to be determined at the sensor. Besides the scaling, this connection is used to compute the compensation of undesirable lateral effects, in particular those to water (H2O) in the form of water vapor in the exhaust gas.
A method for correcting the influence of H2O on the signal of an NH3 gas sensor is known from German Patent No. DE 199 07 669 C1. The reference suggests determining a so-called zero-value function of the gas sensor, which is then used to correct the measuring values during the continued operation. In order to determine the zero-value function, the gas sensor signal is measured during several different operating phases of the engine while the engine is running, without feeding NH3 to the catalytic converter system. The H2O concentration in the exhaust gas, the so-called moisture equivalent Fxc3x84 (ME), is determined at the same time. The moisture equivalent characterizes the operating state of the engine and is determined from the fuel-mass flow together with the air-mass flow, or from the oxygen partial pressure of the exhaust gasxe2x80x94respectively also in combination with a moisture sensor for determining the H2O concentration in air that is suctioned inxe2x80x94or with a moisture sensor in the exhaust-gas flow. In all cases, the ascending gradient and the axial section of the zero-value function, which is approximated as a straight line, is determined from the measured values. In order to correct the measured values of the NH3 sensor during operation, the respective zero-value function is determined and the measured sensor value is then corrected accordingly.
However, the known method has a number of disadvantages.
The delayed response (response threshold) of the NH3 sensor for low NH3 concentrations is not taken into account. In the same way, the influence of the NOx gas components on the response threshold and the NH3 sensitivity are not taken into account. When determining the zero-value function, a linear approximation of the dependence between measured value (Cp) and NH3 concentration is made, which provides acceptable results only for NH3 concentrations up to approximately 50 ppm. The adsorption and conversion behavior of the NH3 sensor, which leads to a distortion of the gas composition arriving at the sensor, is also not taken into consideration.
Thus, it is the object of the invention to provide a method for correcting the signal from a NH3 sensor, which method provides an exact value for the measured NH3 concentration, even with low NH3 concentrations. At the same time, the method takes into account the adsorption behavior and the conversion behavior of the NH3 sensor.
The above object generally is achieved according to the invention by a method for determining the reducing agent concentration (NH3) in the exhaust-gas flow of an internal combustion engine with a (zeolitic) NH3 gas sensor that supplies a base measuring value, wherein the base measuring value is initially corrected by an offset value and by a correction value that depends on the H2O concentration of the exhaust gas to form an intermediate value. Following this, the intermediate value is corrected by an additional value that depends on the NOx concentration of the exhaust gas to obtain a corrected NH3 measuring value.
The values for the dependence of the measured values on the NOx concentration and the NH3 concentration are recorded in a 2-dimensional table.
The two adjacent columns in the table, which best match the given NOx concentration, are selected to determine the NH3 concentration. Following that, the line that best matches the first intermediate value is determined in the first selected column and the final measured value is then determined through interpolation, initially between the columns and then between the lines.